1. Field of the Invention
The present invention relates in general to a hybrid drive system for driving a motor vehicle, and more particularly to an apparatus for controlling a controllable device disposed between an engine and an electric motor as a drive power source and vehicle drive wheels, so that the controllable device is controlled with a reduced shock.
2. Discussion of the Related Art
For driving a motor vehicle, there is known a hybrid drive system including, as a drive power source, an engine operated by combustion of a fuel and an electric motor for a motor vehicle operated with an electric energy, and further including a controllable device such as an automatic transmission, which is disposed between the drive power source and drive wheels of the vehicle. An example of such a hybrid vehicle drive system is disclosed in JP-A-7-67208.
In such a hybrid vehicle drive system, at least one of the engine and the electric motor is selectively operated to drive the motor vehicle in a selected one of a plurality of different running modes, depending upon the running or operating condition of the vehicle, so as to minimize the fuel economy or exhaust gas emissions from the engine. The running modes of the vehicle correspond to respective operation modes of the hybrid drive system, which includes: a an engine drive mode in which the vehicle is driven by the engine; a motor drive mode in which the vehicle is driven by the electric motor; an engine.multidot.motor drive mode in which the vehicle is driven by both of the engine and the electric motor; and an engine drive+charging mode in which the vehicle is driven by the engine while the electric motor is operated as an electric generator for charging an electric energy storage device.
The automatic transmission, which is a controllable device in the hybrid drive system, may be an automatic transmission which has a plurality of operating positions having different speed ratios and which is shifted to an appropriate one of the operating positions by selective engagement and disengagement of coupling means such as clutches and brakes. Alternatively, the automatic transmission may be a continuously variable transmission whose speed ratio can be changed continuously. The automatic transmission may be controlled on the basis of an input torque thereof.
For instance, the automatic transmission having two or more operating positions is shifted from one position to another by engaging one coupling means while at the same time releasing another coupling means. This shifting action of the transmission is referred to as "clutch-to-clutch shifting action", where appropriate. In a motor vehicle equipped with an automatic transmission and using an engine as the sole drive power source, it is widely practiced to estimate the input torque of the transmission, and control the engaging force of the coupling means depending upon the estimated input torque, so as to reduce a shifting shock of the transmission when the clutch-to-clutch shifting action takes place. Such a technique is disclosed in JP-A-3-176240, JP-A-5-77660, JP-A-5-164233 and JP-A-5-296323.
However, inaccurate estimation of the input torque of the automatic transmission results in inadequate shifting actions of the transmission with a shifting shock. The conventional hybrid vehicle drive system suffers from a shifting shock of an automatic transmission due to a variation in the input torque in the different operation modes in which different drive power sources are used.
The clutch-to-clutch shifting action of the automatic transmission may be controlled by a learning control technique, wherein the engaging force or forces of one or both of the two coupling means for effecting the present clutch-to-clutch shifting action is controlled on the basis of the engaging force learned in the past clutch-to-clutch engaging actions, so as to accommodate differences of the hydraulic shift control device between the hybrid drive systems of the individual motor vehicles, and chronological changes of the hybrid drive systems. Where the coupling means is a hydraulically operated frictional coupling device such as a brake or clutch, the learning control of the engaging force of the frictional coupling device is effected by learning control of the transient hydraulic pressure applied to the frictional coupling device during the clutch-to-clutch shifting action of the automatic transmission.
In the conventional learning control of the clutch-to-clutch shifting action of the automatic transmission, the engaging force of the coupling means is controlled in the same learning control method or manner, irrespective of the currently selected operation mode of the hybrid drive system (currently selected running mode of the vehicle). The conventional learning control therefore suffers from inadequate control of the engaging force of the coupling means due to variations in the inertia torque and output characteristics of the hybrid drive system depending upon the currently selected operation mode, whereby the clutch-to-clutch shifting action tends to have a high possibility of giving rise to a shifting shock of the transmission.
There is also known a hybrid drive system for a motor vehicle having front and rear drive wheels, wherein a torque distributing mechanism is disposed between the drive power source and the drive wheels, for controlling a ratio of distribution of a drive torque of the drive power source to the front and rear drive wheels. The torque distributing mechanism may be a center differential device using a planetary gear set, or a differential gear device of bevel gear type. The planetary gear set of the center differential device includes three rotary elements rotatable relative to each other and a differential limiting clutch connecting the two elements of the three rotary elements. The ratio of distribution of the drive torque by the torque distributing mechanism may be controlled by a learning control technique.
In the conventional learning control of the drive torque distribution to the front and rear drive wheels, the torque distribution ratio is controlled in the same learning control method or manner, irrespective of the currently selected operation mode of the hybrid drive system (currently selected running mode of the vehicle). The conventional learning control therefore suffers from inadequate control or change of the torque distribution ratio due to variations in the inertia torque and output characteristics of the hybrid drive system depending upon the currently selected operation mode.